I don't have the reference handy (it's in the hands of some of my SCA friends, and the group I'm currently in is still hunkered down for the winter). However -

Women were a great deal more common on medieval battlefields than is generally made out to be the case. I am still quite certain that female soldiers were anomalies, but there are enough documented accounts of women who were soldiers to know that they were frequent enough that one should not have been shocked to encounter one (surprised, but not shocked). This said, off of the field of the pitched battle, women were common in medieval armies, where they performed tasks that in a modern army would be referred to as logistics. And these women would have been armed: they would have been involved in skirmishes, even if they were trying to avoid them. Eowyn of course was fighting on a pitched battlefied, so in her case, she was still one of the anomalous few. But the defenses of Thranduil's and Galadriel's realms probably included a great many women, first because it was a wood-skirmish type battle and second because these are Elves, who are less sexually dimorphic than Humans.

It is impossible to put numbers on how many women there were on medieval battlefieds. Modern historical research of course loves to find references to women in the records, and historical authors would have considered them noteworthy, because it certainly was against the mold of society. As a result, real statistical testing would be quite impossible. In the SCA, women on battlefields is somewhere in the ballpark of 10%, but the SCA is extremely egalitarian sex-wise, and the dead can still have children in SCA combat, so two of the major reasons for keeping women off of the battlefield don't apply in this mock-combat. If I had to guess a number, I would say that the number of Eowyn-like women who fought alongside men in male-dominated armies in the medieval period is somewhere between 1/100 and 1/1000, but I'm just arm-waving here with no real observational basis.

One last clarification on polygamy: Two relatively recent "western" religions, one being Islam (okay, 1400 years ago is old, but not as old as Taoism), and the other Mormanism (which really is young), allow multiple wives. In both cases, however, I believe that the limit is four wives per husband. One might take this to mean that a man really can't care for more than four wives or the families of more than four wives. If this is the case, then once you've lost three-fourths of your breeding-age men, you should start including women in your armies, because if the ratio of women to men is more than 4:1, you're going to have women who cannot be married, even if you do allow polygamy.